The present invention relates to a graphics display system and method.
Some prior art systems utilizing graphics displays have used a direct view storage tube display, which stores a graphic image directly on the face of a cathode ray tube, so that the image does not have to be continuously refreshed. This approach results in a high-resolution, flicker-free image. In addition to the stored image, a graphics cursor is continuously displayed in a write-through mode, so that it does not become a part of the stored image. By using a graphics tablet or digitizer as an input device, a user can point the cursor at objects on a screen and issue editing commands to the system to alter these objects or add new ones. This type of display has proved adequate for the vast majority of applications in such areas as integrated circuit and printed circuit design, cartography and three-dimensional drafting designing and manufacturing.
A problem with direct view storage tube displays is that an image cannot be selectively erased, since to alter a graphics image requires the erasing of the entire old image and redrawing an entire new one.
To overcome this problem, some prior art systems have employed calligraphic, or vector-stroking, displays continuously refreshed from a list of graphic vectors stored in a vector memory. In such a vector-stroking display, the display reads X-Y coordinate data and intensity information from the memory and strokes the indicated line segments onto the screen in connect-the-dot fashion. When vector data representing a graphic image is altered in the memory from which the display is refreshed, its image on the screen rapidly disappears and the altered portion of the image simultaneously appears, while the remainder of the image remains unchanged.
A problem with such a vector-stroking display is that the complexity of the image which can be displayed without perceptible flickering is fundamentally limited by how far the display tube's electron beam has to travel, how rapidly the beam can be deflected and modulated, and how rapidly the image disappears from the screen.
A display which refreshes the image from a raster memory (also known as dot matrix) avoids such problems of vector stroking. Flicker-free images can easily be generated regardless of the complexity because the electron beam always travels the same path, namely a top to bottom sequence of closely spaced left to right lines, as in a commercial television set. The raster memory is used only to modulate the intensity of the beam.
A problem with raster memories is that once data has been rasterized, there is no good way to deal with the resulting dots in the raster memory. If it is desired to remove only the dots corresponding to a given vector, one could rasterize the vector again and use the resulting sets of dots to erase the raster memory selectively, which would generally remove too many dots. It is desirable to remove only those dots which a particular vector was solely responsible for inserting, and to leave alone those dots which were also inserted by intersecting vectors, which is difficult if not impossible, since in a raster memory all dots look alike.
As a result, after a particular vector is removed, all conceivable intersecting vectors are rewritten into the memory. In a worst case this amounts to re-rasterizing of the entire vector image, which runs the risk of nullifying the reason for going to a refreshed display in the first place, namely the ability to alter the image rapidly. In view of the above background, there is a need for an improved graphics display system and method which provides both vector memory and raster memory capabilities without the above-mentioned limitations.